One technology for aftertreatment of diesel engine exhaust utilizes SCR to enable known chemical reactions which convert NOx into nitrogen (N2) and water (H2O), two constituents found in abundance in earth's atmosphere. A reaction may occur between only ammonia (NH3) stored on surface sites of an SCR catalyst and NOx in the exhaust or a reaction may involve those two reactants and an additional reactant, oxygen (O2), if the latter is also present in the exhaust.
Ammonia which is used in those reactions is created by chemical reactions involving DEF, an aqueous mixture of urea and deionized water (for example, 32.5% urea and 67.5% deionized water), which is injected into the aftertreatment system. When DEF is injected into engine exhaust, exhaust heat vaporizes the water and decomposes the urea. One of the products of urea decomposition is NH3 whose molecules attach to catalytic sites on washcoat surfaces of the SCR catalyst and are available to reduce NOx in exhaust passing across those surfaces by chemical conversion to N2 and H2O.
One factor in the ability of a properly functioning exhaust aftertreatment system to reduce NOx in sufficient quantity for compliance with specified NOx emission criteria depends on use of DEF whose formulation is compliant with a DEF performance standard which defines a correlation of quantity of NOx reduced with quantity of DEF injected. For example, a DEF performance standard may be defined by a DEF manufacturer specifying certain characteristics of its DEF formulation such as urea/water percentages and shelf life. Specifying shelf life is intended to avoid use of DEF which has been stored for a length of time beyond which the formulation may begin to degrade and therefore lose its effectiveness in reducing NOx. Adding a liquid diluent such as water to a DEF manufacturer's formulation reduces the percentage of urea in the mixture and that reduces the DEF's effectiveness in reducing NOx.
In a vehicle which is propelled by a diesel engine, such as a commercial truck for example, a urea quality sensor may be installed in an on-board DEF storage tank to monitor DEF quality. The urea quality sensor monitors the DEF formulation for compliance with a performance standard which correlates quantity of NOx reduced with quantity of DEF injected. One purpose of a urea quality sensor is to detect diluted DEF in the storage tank. DEF may be diluted, either intentionally or unintentionally, when the vehicle is stopped and the storage tank is refilled. When diluted DEF is detected by a urea quality sensor, a fault is logged in an on-board diagnostic system, and a warning alert may be given, such as by illumination of a warning light. Accuracy of a urea quality sensor has an approximate range of +/−3%.